Always send 2 Copper over 1 Estate. Thin faster.
that's not a good advise I think (which is unusual coming from you). It highly depends on the board.
That can be attached to any advice! But sending 2 over 1 is a good rule of thumb, like trashing all starting cards in your hand with Chapel. If you think it's not uncommon for sending 1 Estate to be better, can you elaborate on when/how that is?
yes. it mostly depends on which 5$'s are in the kingdom. if, for example, stables is there, you should almost always return one estate, because the extra economy is worth having more cards. You will hit 5$ a lot, buy stables, cycle through your deck, and win. if the only 5$'s are things like merchant ship, then return 2 coppers. having a lot of economy isn't going to do much for you, your opponent is just going to have a small deck and will crush you, because you can't utilize your economy. basically, the key thing is whether or not the 5$'s are going to help you with the amb war in the long run.
I think returning estates is correct in about half of all games, so it's not an edge case.
I still think it is more of an edge case. Stables is a great example of when returning the Estate is better, because Stables makes Copper much less junky and it gives excellent cycling, letting you play Ambassador more often. But I think most boards will still favour returning 2 cards over 1, at least for the early game.
If you feel adamant about it, I think this would be an excellent article topic. The two Ambassador articles both favour returning 2 Copper over 1 Estate. The
first article by theory (2010) unequivocally states that "Ambassadoring 2 Coppers (rather than 1 Estate) is the best play". The
second article by WanderingWinder (2013) devotes an entire section to the question. The answer is still "usually 2 Copper over 1 Estate" but WW does talk about what situations would warrant returning the single Estate instead. They are exceptions rather than the rule though.
Until now, I hadn't seen any advice contrary to this (at least from prominent players). So if you feel it goes 50/50 over all games, I (and probably the community) would be really interested in seeing your thoughts on it.